1. Field of the Invention
This invention relates to a variable inductance coil device such as a transformer or a choke coil.
2. Description of the Prior Art
For a magnetic core which is used in a transformer or a choke coil, an E-E type (Japanese Patent Publication No. 50372/1980), an E-I type (Japanese Patent Publication No. 24363/1981) and a drum type have been conventionally well-known in the art.
In the E-E type magnetic core, a pair of E-shaped cores made of magnetic material such as ferrite is positioned so that each leg of the cores is opposed each other, wherein a gap is provided between each end of the center legs in order to prevent magnetic saturation. The E-I type magnetic core combines an E-shaped core and an I-shaped core, wherein there is a gap provided on the end of the center leg of the E-shaped core. The drum type core literally uses the drum-shaped core.
However, a method for winding wire around the above-mentioned magnetic core having the gap has frequently caused inductance errors which are induced by dimensional errors in the magnetic core, dimensional errors caused during manufacturing of the gaps, and errors in magnetic permeability of the core. For example, if a choke coil has an effective permeability of around 100, the errors of the inductance is .+-.21% in the E-E type and .+-.16% in the E-I type.
In case of the drum-type magnetic core, the inductance error is relatively small for .+-.6%. However, as illustrated in a diagram of FIG. 7 showing distribution of leakage flux (unit in the diagram is expressed in gauss), the leakage flux near the drum core turns out to be very large, about 20 gauss.